The Lib Dems and Conservatives are set for a Commons showdown over a bid to ease housing benefit changes.

Lib Dem Andrew George is urging MPs to back a plan to exempt more people from what critics call the "bedroom tax" and ministers the "spare room subsidy".

Lib Dem MPs will get behind Mr George's bill, with Labour also promising to back it.

The Tories are understood to have ordered their MPs to vote against the bill on Friday on a three-line whip.

Mr George is outlining what he has called "modest reforms" to the system, with more MPs in the Commons chamber than usual for a Friday.

If his private member's bill becomes law people who could not be found a smaller home would be exempt from the housing benefit changes.

The bill would also exempt disabled people who need a spare bedroom or who have adapted homes.

'Fairer'

It is only the second time the coalition partners have split in this way, with MPs from the two parties being encouraged to vote against each other. The first was over changes to the boundaries of parliamentary constituencies.

Labour MPs will not be whipped in Friday's vote but Mr George hopes enough of them will be in the Commons chamber to ensure his bill gets through to its next stage.

Earlier this week, Labour's shadow work and pensions secretary Rachel Reeves said her party had decided to get behind Mr George's Affordable Homes Bill after initially denouncing the "hypocrisy" of the Lib Dems for attacking a policy which they had voted for.

In July, Lib Dem cabinet minister Danny Alexander said his party would "make the case for these new fairer rules" in government, adding: "If we cannot convince our Conservative coalition partners, we will commit to these reforms in the our 2015 Liberal Democrat manifesto."

'Cynical'

He said the U-turn had been prompted by evidence that the policy was not working.

But the Conservatives accused their coalition partners of a "cynical PR stunt" and insisted the changes were working and were bringing "the bloated Housing Benefit bill under control".

Analysis by BBC Parliamentary Correspondent Mark D'Arcy

Labour see the bill as an opportunity to get the Lib Dems to disown the so-called "Bedroom Tax" (and quite a few Lib Dem backbenchers will be keen to do so) and will therefore support it, "as a rope supports a hanging man".

But with the Tory awkward squad expected to turn out in force to spin out the debate until time runs out ("does the Pope have a balcony?" one awkward squaddie responded, when asked if he would be there) a hundred MPs will be needed to close the debate and ensure the Bill receives a Second Reading. Read Mark's full analysis.

Mr George, who voted against the housing benefit changes, said he welcomed a promise by Labour to amend his legislation to axe benefit cuts completely for people deemed to have too many bedrooms, if the bill moved to its next Commons stage.

He told BBC News he was reasonably optimistic that his bill would get a second reading, due to the "enthusiasm amongst those people who want to support it".

But he conceded there was "equal enthusiasm among Tories trying to stop it", adding: "The government is split on this issue".

He is also concerned that many Lib Dem and Labour MPs who might otherwise support the bill will be in Scotland taking part in the referendum campaign.

He rejected claims that the Lib Dems were only distancing themselves from the policy, which they helped introduce, because of the proximity of the general election.

"What the government found as a result of the implementation of the policy is that there are consequences which are unacceptable and the reason I have brought forward these entirely moderate measures is to deal with those consequences," he said.